Abstract: Electroslag remelting (ESR) is an important secondary refining technique that effectively removes impurities and nonmetallic inclusions, improves the solidification structure of ingots, and enhances the mechanical properties of steels. However, when steels contain easily oxidizable alloying elements such as Al, Ti, Si, B, and rare earth elements (REEs), strong chemical reactions can occur between these elements and unstable components in the CaF₂–CaO–Al₂O₃-based ESR type slag system. These reactions take place in the slag–metal interface and result in uneven distribution of the alloying elements along the height of the remelted ingots, ultimately compromising the mechanical properties of the steels. To reduce the oxidation loss of these alloying elements during the ESR process, a precise design of the CaF₂–CaO–Al₂O₃-based ESR slag composition is crucial. This design facilitates accurate control of the reactive alloying elements within their target composition range, which relies on the feasibility of calculating the thermodynamic activities of components in the slag and alloys employing ion–molecular coexistence theory and Wagner equation, respectively. In addition to incorporating the corresponding oxide additives into the ESR-type slag system to prevent oxidation loss of alloying elements in the electrode during the ESR process, the effects of common components such as CaO and Al₂O₃ in the ESR-type slag, as well as temperature, on the alloying element content can vary. For instance, controlling the Al and Ti contents in alloys is influenced by the combined effects of the CaO composition range and the remelting temperature, in addition to the presence of Al₂O₃ in the ESR-type slag. For B-bearing steels, the B content in alloys can be primarily controlled by the CaO content rather than by Al₂O₃. In the case of alloys containing REEs, such as La, Ce, and Y, the addition of CaO enhances the yield of these elements, while the addition of Al₂O₃ has a negative effect. The accuracy of mass transfer models during ESR not only relies on precise estimation of the thermodynamic activities of components in the slag and molten steel but also on factors such as temperatures at different reaction locations (e.g., electrode tip, metal droplet, interface between slag bath, and metal pool), mass transfer coefficients, and geometric parameters. However, the parameters above are substantially influenced by various factors, such as different ESR operation conditions, slag compositions, and steel grades. Due to the challenges in determining reaction temperatures and fluid flow within the ESR furnace, precisely estimating the mass transfer coefficients of the relative elements in the slag and metal phases at different reaction locations is difficult. Thus, kinetic studies of reactive elements are relatively scarce compared to thermodynamic analyses. Additionally, the physical parameters of the slag system play a crucial role in determining the surface quality and solidification characteristics of the ingots. Current research on the physical parameters of remelting slag systems containing TiO₂, SiO₂, B₂O₃, and rare earth oxides has primarily focused on viscosity and crystallization behavior. However, laboratory studies on the activities of these components remain limited. As the development of low-fluorine ESR-type slag systems attracts increasing attention, the need for relative fundamental research, specifically on the thermodynamics and physicochemical properties of low-fluorine slags, also rises. This research is essential for effectively controlling the contents of reactive alloying elements in the ESR remelted ingots.